Cable Jointing Guide — LV and MV Jointing for Electricians

Types of Joint by Application

• Straight joint: Joins two cable ends in line. The most common type for underground cable extensions and cable repairs.
• Tee joint: Branches off a through-cable to a tee connection. Used in distribution networks and sub-mains from mains cable.
• Termination: Seals the end of a cable for connection to switchgear, transformer, or distribution board. Heat shrink termination kits are the standard solution for LV and MV cables.

Heat Shrink Jointing Systems

• Polyolefin tubing shrinks to grip the cable when heated with a hot air gun at 120 to 200\u00b0C. Multi-layer MV systems include stress control, insulation, and outer protection tubes applied sequentially.
• Advantages: Well-established technology, lower material cost, wide range of cable sizes covered, joints can be made in field conditions if a suitable heat source is available.
• Limitations: Requires a gas torch or hot air gun — not suitable near flammable materials or in confined spaces with poor ventilation. Overheating can damage the cable insulation beneath the joint.
• Key manufacturers: 3M, Tyco Electronics (now TE Connectivity), Raychem — all supply comprehensive heat shrink jointing kits with installation instructions that must be followed precisely.

Cold Shrink Jointing Systems

• Silicone rubber components pre-expanded onto a rigid support tube. Remove the tube by pulling a spiral cord — the silicone contracts to grip the cable without any heat source.
• Advantages: No heat source required — safe in flammable atmospheres, confined spaces, and underground chambers. Silicone provides excellent dielectric properties and long service life. Faster installation in adverse conditions.
• Limitations: Higher material cost, larger installed size than heat shrink, may not suit cables with irregular profiles or unusual outer diameters.
• Key manufacturers: 3M Cold Shrink range, nkt Cables JCS jointing systems, Prysmian Group cold shrink kits.

• Scotchlok IDCs: A metal contact pierces the cable insulation to make contact with the conductor — no stripping required. Suitable for solid or stranded conductors up to approximately 2.5mm\u00b2 depending on product range. Widely used in telecom, data, and low-current control wiring.
• Crimp ferrules: Conductors are inserted into a copper or aluminium ferrule and compressed with a ratchet crimping tool calibrated for the ferrule size. Provides a gas-tight connection that resists corrosion. Essential for aluminium conductors where screw-type connectors would cause cold flow and loosening.
• Mechanical connectors: Split-bolt, Polaris multi-tap, and proprietary maintenance-free connectors for LV distribution cables up to 300mm\u00b2. Suitable for copper or aluminium conductors with appropriate bi-metallic connectors where required.

• Outer sheath removal (LV SWA/XLPE): Use a rotary cable stripper set to cut through the outer sheath without penetrating the armour or bedding. Score a ring cut to the required strip length, then make a longitudinal cut and peel back the sheath. Never use a craft knife around the circumference of the cable.
• Armour removal: Cut individual armour wires with side cutters one at a time, or use armour cutters for larger cables. Bend wires back carefully to avoid damaging the cable bedding. Leave sufficient armour for connection to the earth clamp.
• Core insulation removal: Use a calibrated wire stripper set for the conductor cross-section. Automatic or ratchet strippers provide the most consistent strip length and avoid nicking conductors. For XLPE insulation, a thermal wire stripper provides the cleanest cut.
• Semi-conducting layer (MV cables): Use a purpose-made semi-con stripping tool. The semi-con must be removed cleanly to a precise tapered pencil end — any residue left on the XLPE insulation will cause electrical stress concentration and eventual joint failure.

Fault Location to Joint Repair Sequence

• Step 1 — Isolate and prove dead: Isolate the affected circuit and confirm it is dead using an approved voltage indicator. Co-ordinate with the network operator for DNO-owned cables.
• Step 2 — Insulation resistance test (megger): Perform IR tests between all conductors and to earth to confirm the fault type (phase-to-earth, phase-to-phase, open circuit) and verify it is permanent. Use 500 V DC for LV cables. Record all readings.
• Step 3 — TDR fault location: A time-domain reflectometer (TDR) sends a pulse along the cable and measures the time to the reflected return, giving an estimated fault distance. For buried HV or LV networks this narrows the excavation zone to a few metres. Typical fault-location duration for HV/LV underground networks is 4 to 6 hours, including megger confirmation and TDR pin-pointing.
• Step 4 — Excavate and inspect: Excavate at the TDR-predicted location. Use a cable tracer or signal generator to confirm the route before digging. Visually inspect the cable for physical damage — third-party excavation strikes are the most common cause of permanent underground faults.
• Step 5 — Joint repair and retest: Cut out the damaged section, prepare and joint with an approved kit, then retest IR and continuity before reinstatement. Mark the joint location per Regulation 522.8.10 (marker tape above the cable) and record on as-installed drawings.

• Test voltages and minimums (BS 7671 Reg 643.3.2, Table 64):
  • SELV / PELV circuits: 250 V DC — minimum 0.5 M\u03a9
  • Circuits up to 500 V (excluding SELV/PELV): 500 V DC — minimum 1.0 M\u03a9
  • Circuits above 500 V: 1,000 V DC — minimum 1.0 M\u03a9
  Always use 250 V DC when jointing PELV control or data cables to avoid equipment damage. Disconnect all electronic devices before testing at any voltage.
• Practical interpretation: Values below 10 M\u03a9 on a new joint warrant investigation even though the regulatory minimum is 1.0 M\u03a9. Well-made joints on new cable routinely measure hundreds or thousands of M\u03a9.
• Testing methodology: Test between each conductor pair (L1-L2, L1-L3, L2-L3, L1-E, L2-E, L3-E). For three-phase cables with neutral, test N-E and each phase to neutral as well. Record all readings on the test schedule.
• Resin cure time: If a resin-poured enclosure is used, allow the resin to fully cure per the manufacturer instructions before final IR testing. Testing during cure can give misleadingly low readings.

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  — issue minor electrical installation works certificates for cable repairs and joint work, including pre- and post-repair insulation resistance readings.
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  — generate a compliant EIC for new underground cable installations including jointing details and test schedules.
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  — document inaccessible or defective joints identified during periodic inspection as C2 or C3 observations with photographic evidence.